Various electronic applications exist that involve sending varying currents through a circuit and then reading and recording the output voltage that corresponds to each current. In many cases, this output voltage is the base-emitter voltage, a p-n junction, of a bipolar junction transistor (BJT). One such circuit is an electronic temperature sensor circuit that is configured to measure the temperature on a remote (separate) silicon chip by providing two target collector currents (IC1, IC2) to a p-n junction located on the remote chip. This circuit measures two diode voltages (VBE1, VBE2) of this p-n junction and processes the diode voltages to determine the actual temperature at the remote location. Most p-n junctions employed for this purpose are parasitic vertical p-n-p silicon based transistors. Also, the temperature sensor circuit is usually arranged to control the emitter currents of the transistor.
The classic diode equation determines a change in the base emitter voltage (ΔVBE) for a p-n-p transistor as follows:
                              Δ          ⁢                                          ⁢          Vbe                =                  η          ⁢                                    κ              ⁢                                                          ⁢              T                        q                    ⁢                      ln            ⁡                          (                                                I                                      C                    ⁢                                                                                  ⁢                    1                                                                    I                                      C                    ⁢                                                                                  ⁢                    2                                                              )                                                          (        1        )            where η is a non-ideality constant substantially equivalent to 1.00 or slightly more/less, κ is the well known Boltzmann's constant, q is the electron charge, T is the temperature in Kelvin, IC1 is a first collector current, and IC2 is a second collector current that are present at the measurement of a first base-emitter voltage and a second base-emitter voltage.
The classic diode equation is often employed to determine the actual temperature at a remotely located p-n-p transistor based on a ratio of approximated collector currents. In the past, since a ratio of collector currents tended to be relatively equivalent to a ratio of known emitter currents (IE), the diode equation could be accurately approximated in a rewritten form that follows:
                              T          =                                    Δ              ⁢                                                          ⁢                              V                BE                                                    η              ⁢                              κ                q                            ⁢                              ln                ⁡                                  (                                                            I                                              E                        ⁢                                                                                                  ⁢                        1                                                                                    I                                              E                        ⁢                                                                                                  ⁢                        2                                                                              )                                                                    ;                              where            ⁢                                                  ⁢                                          I                                  C                  ⁢                                                                          ⁢                  1                                                            I                                  C                  ⁢                                                                          ⁢                  2                                                              =                                    I                              E                ⁢                                                                  ⁢                1                                                    I                              E                ⁢                                                                  ⁢                2                                                                        (        2        )            
However, due in part to process variations for integrated circuits with smaller process geometries, the assumption regarding relatively equivalent ratios may no longer be valid. The beta (ratio of collector current over base current) has been shown to vary as much as ten percent or more between two known emitter currents for p-n-p transistors in integrated circuits manufactured from relatively smaller process geometries. Thus, the diode equation approximation (Equation 2) regarding the ratios of collector and emitter currents for a transistor can cause relatively inaccurate temperature measurements in an integrated circuit based on smaller process geometries. Relatively significant inaccurate temperature measurements can occur in integrated circuits that have process geometries of 90 nanometers or less. It should be appreciated that these measurements represent examples of problems experienced, and different manufacturers may start showing these effects at different process geometries.
Subsequent art provided for a more accurate temperature measurement for a transistor with a rewritten form of the diode equation (Equation 3) that provides for actually measuring or controlling the ratio of collector currents instead of the ratio of emitter currents.
                    T        =                              Δ            ⁢                                                  ⁢                          V              BE                                            η            ⁢                          κ              q                        ⁢                          ln              ⁡                              (                                                      I                                          C                      ⁢                                                                                          ⁢                      1                                                                            I                                          C                      ⁢                                                                                          ⁢                      2                                                                      )                                                                        (        3        )            
The disadvantage of this method, however, was that it required measuring IC and converting it to a digital form in real-time, which, when done accurately, is extremely expensive.
Yet another alternative has been to drive the collector currents to a predetermined ratio, thus eliminating the need to measure the collector currents independently. Consequently, Equation 3 can be rewritten as:
                    T        =                              Δ            ⁢                                                  ⁢                          V              BE                                            η            ⁢                          κ              q                        ⁢            ln            ⁢                                                  ⁢            N                                              (        4        )            
Previously, this has been accomplished by using a simple multiplexer that switches between a first current source and a second current source. The disadvantage to this method is that switching between two independent currents sources introduces transistor mismatch. In other words, the threshold voltage (Vt) associated with each current source may be mismatched. Furthermore, the circuit must account for two different overdrives. Thus, the variations in threshold voltage and overdrive cause deviations from the desired ratio.